Various means and methods are known for projecting images from phosphor coated cathode ray tubes for standard television. However, it should be appreciated that these standard phosphor CRT displays do not emit sufficient light for high contrast projected images. Further, if the emission intensity of a phosphor CRT display is increased to improve contrast, a loss in resolution results.
Recognizing this shortcoming of phosphor CRT displays, cathodochromic CRT projection displays have been developed, such as disclosed in U.S. Pat. No. 3,959,584 issued to Todd, Jr. Using a cathodochromic CRT (CCRT) with any standard Group III facsimile machine and a speaker phone, individuals have the capability of scanning, transmitting, displaying and annotating virtually any standard size document in a simple-to-use conferencing environment. In contrast to phosphors, cathodochromic materials have the property that they do not emit light but rather change color when excited by an electron beam beyond some threshold. In the case of cathodochromic bromine sodalite, some of the coloration can be bleached by visible light, optical mode coloration, but the remaining coloration, thermal mode coloration, remains indefinitely. The material is heated to approximately 300.degree. C. to cause erasure. Since the induced thermal mode coloration is unaffected by exposure to visible light, the images on the cathodochromic image screen of a cathodochromic CRT can be projected by either reflecting light from the screen's surface or transmitting light through the screen. In the former case, the device operates in a manner similar to an opaque projector. In the latter case, the screen acts as a light valve passing light in the uncolored areas and absorbing it in the colored areas.
Since cathodochromic CRT displays utilize reflected or transmitted rather than emitted light, cathodochromic CRT displays provide higher resolution images since images can be written using very low electron beam current. As a further advantage, image brightness is much greater since it comes from an external projection lamp, thus providing images that may be viewed in high ambient light conditions.
It should be recognized that the improved resolution and higher brightness images available with cathodochromic CRT displays can only be realized and appreciated when the projection unit is properly aligned with the display screen. When the projection unit is positioned too close to or too far away from the viewing screen, the focus is not sharp and resolution suffers. Further, when the projection unit is positioned at an angle rather than parallel to the viewing screen, the resulting image suffers from distortions due to the different projection distances that projected image must travel to reach the screen. Thus, it should be appreciated that proper alignment of the projection unit with the viewing screen is critical.